The present invention relates to a process and apparatus for the preparation of single silicon crystals having a reduced level of contamination. More specifically, the present invention relates to a process and apparatus for the preparation of single silicon crystals wherein the structural graphite components in the crystal growth chamber of a Czochralski crystal pulling apparatus have been coated either with two protective layers including a first protective layer such as silicon carbide or glassy carbon, and a second protective layer of silicon, or coated with a single protective layer comprising a mixture of silicon carbide and silicon.
Single crystal silicon which is the starting material for most processes for the fabrication of semiconductor electronic components is commonly prepared with the so-called Czochralski process. In this process, polycrystalline silicon ("polysilicon") is charged to a crucible, the polysilicon is melted, a seed crystal is immersed into the molten silicon and a single crystal silicon ingot is grown by slow extraction to a desired diameter.
The crystal pulling apparatus commonly utilized in the Czochralski process contains numerous internal parts surrounding the molten silicon containing crucible. These internal parts are constructed of graphite and generally referred to as "hot zone" parts. These hot zone parts, such as susceptors, heaters, thermal shields, heat reflectors or insulation, control the heat flow around the crucible and the cooling rate of the growing crystal. It has been recognized in the art for some time that although the graphite components used in the crystal pulling apparatus are not in direct contact with the molten silicon or the growing crystal, the use of such components at the high temperatures necessary to melt the polysilicon and grow the resulting crystal can result in the outgassing of particles and resulting high level contamination of the melt and subsequently the grown crystal with molybdenum, iron, copper, nickel, and other unwanted contaminants. It is well known that metals such as iron and molybdenum reduce minority carrier lifetimes in silicon wafers and copper and nickel can lead to oxygen induced stacking faults in the resulting crystal. Also, oxygen produced during the crystal growing process through the interaction of the silicon melt and the crucible which is present around the graphite components can cause the graphite to undergo oxidation and cause the further release of particles from pores in the graphite, as well as weaken the graphite structure and cause the parts to buckle.
In order to reduce the risk of crystal contamination with contaminants which can be outgassed by graphite parts located around the growing crystal, it has been common for all graphite components contained in the hot zone to be coated with a protective barrier layer such as silicon carbide or a glassy carbon coating. Because of its high temperature oxidation resistance, silicon carbide is widely used to coat graphite parts used in the hot zone of a crystal pulling apparatus. Silicon carbide coatings provide a barrier to impurity outgassing by sealing the graphite surface, thus requiring impurities to pass through the coating by grain boundary and bulk diffusion mechanisms. This coating is used to contain unwanted contaminants that are generated by the graphite during the crystal pulling process. The silicon carbide layer is generally on the order of about 75 to about 150 micrometers thick, and covers the graphite surface. One method of depositing a silicon carbide layer over graphite is described by Scheiffarth and Wagner in Surface and Coatings Technology, 54/55 (1992) 13-18.
Similar to the silicon carbide coating the glassy carbon coating on graphite is used to contain unwanted contaminants that the graphite generates during exposure to high temperature. A method of providing a glassy carbon layer over a graphite body is described by Lewis et al. in U.S. Pat. No. 5,476,679.
Although the use of a silicon carbide coating or glassy carbon coating over graphite has reduced the amount of undesirable contaminants entering the silicon melt and/or the grown crystal, neither approach has been successful in totally eliminating the problem of particulate generation by graphite and the resultant contamination the grown crystal. Iron contamination from graphite remains a prominent problem even with the use of a silicon carbide or glassy carbon coating. Undesirable metals such as iron appear able to penetrate these coatings in an amount sufficient to degrade the resulting crystal. Also, it is believed that the typical silicon carbide coating provided by industry is itself contaminated with about 1 ppma iron. When this coating is heated in the silicon crystal growth environment, the iron can diffuse to the surface, evaporate, and become attached to the growing crystal.
Therefore, a need still exists in the semiconductor industry for a method which will further reduce the level of contaminants entering the silicon melt during the crystal growing process due to particulates generated from components within the hot zone of the crystal pulling apparatus.